Various related art mobile phone apparatuses have been proposed that include a main telephone unit having a rechargeable battery, and a charger which supplies power to the rechargeable battery. For example, Japanese Patent Application No. JP-A-2006-208595 describes a related art mobile phone apparatus that in a state where the main telephone unit is installed in the charger and a function key is depressed for a designated time or more to make a request for transition to an illumination mode of using light of a backlight of a liquid crystal display screen for illumination, a message indicating the transition to the illumination mode is displayed on the liquid crystal display screen. Then, after an elapse of a predetermined amount of time, a light emission amount of the backlight is increased or a contrast of the liquid crystal display screen is set low to increase a transmission light amount of the backlight that is transmitted through the liquid crystal screen.
However, with the arrangement described in JP-A-2006-208595, in order to increase the light emission amount of the backlight, power supplied to a backlight drive circuit that drives the backlight has to be increased and overvoltage protection has to be provided so that an overvoltage is not applied. The increased power and overvoltage protection are carried out by means of a voltage booster circuit that boosts an output voltage of the rechargeable battery. Overvoltage protection in a booster circuit is high in cost and poor in efficiency.
An alternative method is to supply the supply power of the charger directly to respective devices via a Zener diode without involving the voltage booster circuit. However, this alternative method has a disadvantage in that the voltage of the supply power is unstable, etc.